1. Field of the Invention
This invention relates to an electrically conductive silicone rubber composition and to heating elements made therefrom such as heating tapes and sheets.
It is desired that heating elements are made from a composition having a positive temperature co-efficient of resistance (PTC) of such characteristic that above a predetermined temperature a substantial increase in resistance occurs. As a result, the heating elements are self-limiting in that when the temperature of the element increases above a certain temperature its resistance increases with the result that less current flows with consequent reduction in heat output and hence further increase in temperatures does not occur. A steady state is reached at which the heat produced by the element is balanced by the heat lost by the element.
2. Description of the Prior Art
It has been proposed to use electrically conductive silicone rubber compositions in the manufacture of heating elements such as tapes. For example, in U.K. Patent Specification No. 957,108 it is proposed to utilise, in the manufacture of heating elements, an electrically conductive silicone rubber composition comprising 15% to 48% of carbon black, 4% to 14% of filler, up to 7% of peroxide curing agent and, optionally, up to 7% of structure control additive, the balance being silicone gum. The above mentioned percentages are expressed in terms of weight based on the total material weight. The filler is provided solely to achieve desired mechanical properties such as strength and/or viscosity. Such a composition has not found general acceptance in the manufacture of heating elements because the composition has a PTC of such characteristic that no effective self-limiting effect is achieved in the heating elements made from the material; because the material has a relatively low stability i.e. a relatively high change in conductivity during storage prior to use, a relatively short life i.e. a relatively high change in conductivity during use, further the initial current on switching on the heating element is relatively high which leads to the need to provide switch gear, wiring, fuses etc. capable of withstanding the high switch-on current and because it is difficult to achieve, and to maintain in production, a predetermined and uniform conductivity.